I don't know how we got onto the 53 series, but yes the 6v71 and straight 671 are different in size and displacement. You can not interchange the rotors. Basic Blower Measurements: http://www.wallaceracing.com/Basic%20Blower%20Measurements.htm Also, don't confuse the numbers used to describe the Detroit Diesel engine. Those numbers have nothing to do with the displacement of the blower used in any series. The numbers are used to describe both the number of cylinders and the displacement of each of those cylinders. Example: 6-71 = six cylinders, 71 CID per cylinder IIRC, an "N" after the CID per cylinder is for naturally aspirated, and "T" is turbo charged.